Color Calibration and Gamma Curves

Conventional film is well-known for having color reproduction characteristics that vary over a considerable range. Some films are prized for their vivid, saturated colors and are used to make product shots exciting or to brighten an overcast day. Other films are chosen for their neutrality or realistic flesh tones. Digital color offers new capabilities, but carries with it the same color reproduction concerns that film has long displayed. Whether you're shooting film or pixels, in most cases, you'll want your image to closely approximate the original. Neither color display systems nor color hardcopy output devices are consistent or particularly accurate. The best you can do is calibrate your peripherals so that there is a relationship between the colors in the original, what you see, and what you get as output.

The process is complicated by two facts. First, the response of any color system is rarely linear. And, to make things worse, it's difficult to describe a color in such a way that it means exactly the same thing to everyone. Assume for a moment a 24-bit system with 256 different tones of each color. A value of 0 for a particular color should represent no color; a value of 255 should represent the maximum intensity of that color. On a linear scale, 64 would represent about 25% intensity, 128 would be 50%, and so on. Yet, in real applications, an intensity of 64 is not half that of 128. It corresponds to some other percentage, depending on the characteristics of the device being used. The relationship of the actual representation to the ideal is known as a gamma curve.

Scanners do happen to conform to the ideal rather closely, but computer displays and printers tend to vary greatly. If you know the gamma curve of a particular device, however, you can correct it. For example, if you know that, with a certain device, a value of 64 produces an intensity that is only 90% of what it should be to be linear, you can boost that value by an appropriate amount whenever it occurs.

This is done by building a gamma correction table using the tools supplied by your scanner vendor. The table will include a value for each of the levels used in a system. The correction values can be substituted automatically by your software for the default values, theoretically producing a perfect, 45° gamma curve. Many vendors provide device characterization files for their products. If you're using Windows, you can follow the instructions at the end of this chapter for using the Adobe Gamma application to profile your monitor. Mac OS X users can use their operating system's built-in gamma correction tools instead.